Injection by means of hydraulic dosing pumps of a mixture of water and an auxiliary liquid product in two separate phases is known, in particular from FR 2 967 218 in the name of the applicant company. The auxiliary liquid, in the case of use in an agricultural environment, can be constituted by nutrients in solution, or by veterinary drugs. It is important for the mixing of the main liquid and the auxiliary liquid to be as homogenous as possible. The same requirement may apply in other applications, since the dosing pumps in question can be used in industry, in hospitals, or in other fields which also make it necessary to obtain a homogenous liquid mixture, according to the dose prescribed for the auxiliary liquid.